Period date calculator

Enter the day your last period started and how long your cycle usually is. You get estimated start dates for your next six periods, with a likely range for each, and a calendar file you can add to your phone.

This calculator runs entirely in your browser. The dates you enter are not sent anywhere, not stored and not seen by anyone, including us. Close the tab and they are gone.

Every date here is an estimate from a typical cycle length. It is not birth control and it is not medical advice.

How the dates are worked out

Each estimated start is the first day of your last period plus your cycle length, repeated: with a 28-day cycle, the next periods are estimated 28, 56, 84 days after the last start, and so on. The estimated period days run from that start for your usual period length.

Real cycles vary from month to month, so each estimate comes with a likely range. The range is 3 days either side for the next period and widens by 3 days for each cycle after it, up to 9 days either side from the third period on. This is the same rule the Isla app uses before it has logged cycles of its own to learn from.

The calendar file

The .ics file is made inside this page and saved straight to your device. It holds one all-day event per estimated period, with the likely range in the event notes. Open it on an iPhone or Mac to add the events to Calendar, or import it into Google Calendar or Outlook. If you share a calendar with anyone, add the events to a calendar that is only yours, or choose the "Reminder" name.

When an estimate is less useful

An estimate from one typical length works best when cycles are fairly steady. Recently stopping or starting hormonal birth control, pregnancy, breastfeeding, the years around perimenopause, stress, illness and travel can all move a period by more than the range shown. If your cycles have changed and that worries you, talk with a clinician.

Keeping track over time

Lengths for every cycle from 21 to 40 days are on the cycle length pages, and lookup tables by the month your last period started are on the next period date pages. For keeping cycle dates private, see the privacy guides.